that is not true for communism as a social system, only for the one applied in different regimes that through populism got power.You can see the same modus operandi in the most capitalist country in the world, where the candidacy won through a populism that attributed all the ills of the nation to something external (foreigners).By making this analogy it is explicit that any implemented social system is corrupted by power, and by many more complex things. Therefore it is invalid to compare a social system,with a religion.in that phrase Marx wanted to get the working class to stop believing that it was not necessary to fight for better social conditions,because in the next life they would have everything.